Heat-Resistant 'Nanoglue' Creates Superstrong Bond

Photonics.comMay 2007
TROY, N.Y., May 16, 2007 -- By taking a nanolayer of a commercially available glue material and sandwiching it between a thin film of copper and silica, researchers have developed an inexpensive method of bonding materials that don't normally stick together and dramatically enhanced the glue's heat resistance. The adhesive, which is based on self-assembling nanoscale chains, could impact next-generation computer chip manufacturing and a wide assortment of micro- and nanoelectronic devices.

The project was led by Rensselaer Polytechnic Institute materials science and engineering professor Ganapathiraman Ramanath, who said that, like many key scientific discoveries, he and his team happened upon the heat-hardened nanoglue by accident. A new method allows a self-assembled molecular nanolayer to become a powerful nanoglue by "hooking" together any two surfaces that normally don’t stick well. Unprotected, a nanolayer (green ball: silicon, blue: sulphur, red: carbon, white: hydrogen) would degrade or detach from a surface when heated to 400 °C. But when topped with a thin copper film that binds strongly with the nanolayer, heat causes the nanolayer to form strong chemical bonds to the silica underlayer -- hooking or gluing the copper-silica "sandwich" together. This technique produces a sevenfold increase of the thin-film sandwich’s adhesion strength and allows the nanolayer to withstand temperatures of at least 700 °C. This new ability to bond together nearly any two surfaces using nanolayers will benefit nanoelectronics and computer chip manufacturing. Other envisioned applications include coatings for turbines and jet engines, and adhesives for high-heat environments. (Image courtesy Rensselaer/G. Ramanath)
For years Ramanath has investigated ways of assembling layers of molecular chains between two different materials to enhance the structural integrity, efficiency, and reliability of semiconductor devices in computer chips. His team has shown that molecular chains with a carbon backbone -- ending with appropriate elements such as silicon, oxygen, or sulfur -- can improve adhesion and prevent heat-triggered mixing of atoms of the adjoining substances. Recently, Ramanath’s group and other researchers have found these nanolayers to be useful for creating adhesives, lubricants, and protective surface coatings.

The nanolayers, however, are extremely susceptible to heat and begin to degrade or simply detach from a surface when exposed to temperatures above 400 °C (752 °F). This severe limitation has precluded more widespread use of the nanolayers.

Ramanath’s team decided to sandwich a nanolayer between a thin film of copper and silica, thinking the extra support would help strengthen the nanolayer’s bonds and boost its adhesive properties, but the team found more than it bargained for.

When exposed to heat, the middle layer of the “nanosandwich” did not break down or fall off, as it had nowhere to go. But that was not the only good news: The nanolayer’s bonds grew stronger and more adhesive when exposed to temperatures above 400 °C. Constrained between the copper and silica, the nanolayer’s molecules hooked onto an adjoining surface with unexpectedly strong chemical bonds.

“The higher you heat it, the stronger the bonds are,” Ramanath said. “When we first started out, I had not imagined the molecules behaving this way.”

To make sure it wasn’t a fluke, his team recreated the test more than 50 times over the past two years. The results have been consistent, and show heating up the sandwiched nanolayer increases its interface toughness -- or “stickiness” -- by five to seven times. Similar toughness has been demonstrated using micrometer-thick layers, but never before with a nanometer-thick layer. A nanometer is 1000 times smaller than a micrometer.

Because of their small size, these enhanced nanolayers will likely be useful as adhesives in a wide assortment of micro- and nanoelectronic devices where thicker adhesive layers just won’t fit.

Another unprecedented aspect of Ramanath’s discovery is that the sandwiched nanolayers continue to strengthen up to temperatures as high as 700 °C (1292 °F). The ability of these adhesive nanolayers to withstand and grow stronger with heat could have novel industrial uses, such as holding paint on hot surfaces like the inside of a jet engine or a huge power plant turbine. Along with nanoscale and high heat situations, Ramanath is confident the new nanoglue will have other unforeseen uses.

“This could be a versatile and inexpensive solution to connect any two materials that don’t bond well with each other,” Ramanath said. “Although the concept is not intuitive at first, it is simple, and could be implemented for a wide variety of potential commercial applications.

“The molecular glue is inexpensive -- 100 grams cost about $35 -- and already commercially available, which makes our method well-suited to today’s marketplace. Our method can definitely be scaled up to meet the low-cost demands of a large manufacturer,” he said.

Ramanath and his team have filed a disclosure on their findings and are moving forward toward a patent. The team is also exploring what happens when certain variables of the nanoglue are tweaked, such as making taller nanolayers or sandwiching the layers between substances other than copper and silica.

Ramanath's ongoing research is supported by the National Science Foundation, the US-Israel Binational Science Foundation, the Alexander von Humboldt Foundation, and New York state through the Interconnect Focus Center.

The work is featured in the May 17 issue of the journal Nature. Along with Ramanath, Rensselaer materials science and engineering graduate students Darshan Gandhi and Amit Singh contributed to the paper. Other co-authors include Rensselaer physics professor Saroj Nayak and graduate student Yu Zhou, IBM researcher Michael Lane at the T.J. Watson Research Center in Yorktown Heights, N.Y., and Ulrike Tisch and Moshe Eizenberg of the Technion-Israel Institute of Technology.

An intermolecular substance that serves to hold materials together. Two types are used in the optical industry: one, which must be transparent and colorless, to cement lenses together; and a general-purpose adhesive for bonding prisms and other glass parts to their metallic supports.

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